1. Field of the Invention
The invention relates generally to geophysical instrumentation, and more particularly to instrumentation for electrical resistivity measurements.
2. Description of the Prior Art
The electrical resistivity method is one of the most widely used geophysical methods for investigating subsurface resistivity distribution. In this method, electrical currents are pumped into the ground using two current electrodes, and the resulting potential difference between two arbitrary points is measured by two potential electrodes. The current and potential difference determine the resistivity. The method is easy to implement in the field and the ensuing interpretation is one of the simplest in all geophysical methods. However, the method is adapted only for interval measurements, typically using large plates or spikes for electrodes. It would also be useful to have a system that provides both point measurements and interval measurements.
Accordingly it is an object of the invention to provide a miniaturized electrical resistivity probe for point measurement.
It is also an object of the invention to provide an array of miniaturized electrical resistivity probes that can be used for both point measurements and interval measurements, in both single-borehole and cross-borehole configurations.
The invention is a miniaturized electrical resistivity (ER) probe based on a known current-voltage (I-V) electrode structure, the Wenner array. But the miniaturized probe is designed for local (point) measurement. The electrodes are typically about 1 cm long, separated by 1 cm, so the probe is only about 1 inch long. The electrodes are mounted to a rigid tube (or other structure) with electrical wires in the tube and a sand bag may be placed around the electrodes to protect the electrodes and maintain good contact with surrounding media. The probe may be attached to a packer that is lowered into a borehole and inflated. The electrodes make contact with the surrounding medium. In a dual mode system, individual probes of a plurality of spaced probes can be used to measure local resistance, i.e. point measurements, but the system can select electrodes in different probes to make interval measurements between probes.